DE102008004908A1 discloses a motor vehicle, in which a control device issues a stimulus signal perceptible to the driver, ascertains the reaction time of the driver to the stimulus signal and on the basis of this reaction time or the absence of a reaction assesses the alertness of the driver. Examples for possible stimulus signals are listed, such as a reduction in vehicle speed, a change in the interval duration of a windscreen wiper system, a change in the loudness of an audio system, a change in the illumination of color or brightness of a display instrument, etc. The stimulus signal must not have a direct influence upon the traffic safety of the motor vehicle. A disadvantage of this system consists in that an arbitrary deviation of the vehicle from the defaults set by the driver, be it in the form of a reduction in speed or a windscreen wiper interval, is perceived by the driver as annoying if not appropriate to the current traffic situation, and accepted without reaction if appropriate. Therefore there exists the possibility that the assessment of driver alertness based on such stimulus signals is perceived as an annoying interference and paternalism, or, in case the driver does not feel the need to correct the influence of the stimulus signal upon the vehicle and therefore ignores it, that alertness may be inaccurately assessed.